feat(rust): Pub-licize Expr DSL Function enums

[Shoeboxam]

Changes the visibility of parts of the Expr DSL that I'd like to add support for in OpenDP (see current docs) to pub.

[ritchie46]

They are not meant to be public. They are internal state of the DSL. Could you explain why you need that public?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.00%. Comparing base (9e32651) to head (df10a26).
Report is 102 commits behind head on main.

Additional details and impacted files

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##             main   #20421      +/-   ##
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- Coverage   79.63%   79.00%   -0.63%     
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  Files        1564     1562       -2     
  Lines      217804   220166    +2362     
  Branches     2477     2486       +9     
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+ Hits       173439   173944     +505     
- Misses      43796    45649    +1853     
- Partials      569      573       +4     

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[Shoeboxam]

The use-case is a software project (OpenDP) that proves that a given Polars DSLPlan satisfies differential privacy. For an example of its use, the Switzerland Federal Statistical Office used the tool to release high-resolution poverty statistics.

To accomplish this, I've written a number of Polars plugins, as well as a static analyzer for DSLPlans. The analyzer tracks key properties about the data via a recursive parser over each expression/step in the DSLPlan. Over the last year I've been expanding the kinds of expressions and plans that the analyzer can reason about.

For example, if an expression in the plan contains an ArrayFunction::Explode, then in differential privacy jargon the analyzer needs to account for a k-stable transformation (where k is the static array length). Whereas if the DSLPlan contains an ArrayFunction::Min, then the analyzer needs to account for a 1-stable transformation. Unfortunately, since the array enum is private, I can't add logic to the analyzer for array functions, so the analyzer rejects all Polars plans with array functions.

For more context on the project:

• repository
• recent blog post
• getting-started documentation